I asked one time why they check membership cards at the entrance, and they said it was because they want to make sure you have a card BEFORE you get to the checkout line and hold everything up when you can't buy anything. Sometimes people forget their cards, and sometimes people don't even realize you need a card to shop there. But if you don't have a card, it's a waste of everyone's time for you to shop there; so I can actually see the point of this, given how bad the checkout lines at Costco usually are.
Anyway, it's not really hard to bypass the check. Half the time you can just walk in and they don't say anything anyway. And if you do get questioned, you can say you're going to the pharmacy (by federal law they have to let you fill prescriptions without a membership) or that your spouse has the card and is already in the store. Or even just say you want to get a membership.
I generally find the whole receipt checking concept irritating and waste of time, and I almost never bother showing my receipt at most stores (usually I'll just walk on by with a cheerful "No thanks!" when asked), but for some reason showing my receipt at Costco doesn't annoy me. Partly because it's one of the things you agree to when you sign up for a membership, but also because it means I can leave my cart outside the bathroom while I go and take a whiz after checking out without any fear of anything being stolen from my cart.

After all, if I notice something missing, I can just go grab another one from the shelf, and as long as I haven't left the store yet they can't claim I didn't pay for it.